A rock band's lead singer got a little too hardcore during a live show -- pulling down her pants and taking a major leak on a dude's face ... who seemed to be all about it.
This wild incident went down Thursday night during the Welcome to Rockville festival at the Daytona International Speedway -- where Brass Against performed to a ton of fanfare ... and eventually, to a ton of grossed-out spectators who witnessed this unsanitary scene.